Singapore and Japan, which import huge amounts of Chinese eels, have also stepped up checks for traces of the chemicals.
The Korea Food & Drug Administration (KFDA) said in a statement that it had found the carcinogens malachite green and leucomalachite in some imported Chinese carp available at local wholesale markets, which were supposed to be imported prior to 23 August.
They have quarantined Chinese and Vietnamese eels after finding malachite green in eel and eel-related products from these countries.
Earlier malachite green has been also found in eels and other freshwater fish imported from China, which were supplied to Hong Kong.
Tokyo has warned it will consider a ban if widespread contamination is found, while Singapore will require all eels and freshwater fish imported from China to undergo pre-export tests.
Malachite green, which has been found to be carcinogenic in rats, is one of the main cornerstones of fish disease treatments used for many years against a range of parasites. However the chemical is now banned in many countries, including China.
Some biologists say there is no need to panic over the detection of malachite green in fish because huge amounts of contaminated eel and fish would have to be eaten before one could develop a risk for cancer. But many also advise it is best to play it safe.
Chinese officials provided a list of 18 fish farms in the southern Chinese province of Guangdong that were supposed to be raising safe fish.
"Normally big fish farms are properly managed and do not have such problems. But as you know, there are millions of small family fish farms across China and there are some problems and difficulties in managing them. That's why these cases have emerged," said Ma Weijun, an official at the Fishery Division of China's Ministry of Agriculture.